The cultivated sunflower (Helianthus annuus L.) is a major worldwide source of vegetable oil. In the United States, approximately 4 million acres are planted in sunflowers annually, primarily in the Dakotas and Minnesota.
Field crops are bred through techniques that take advantage of the plant's method of pollination. A plant is self-pollinated if pollen from one flower is transferred to the same or another flower of the same plant. A plant is cross-pollinated if the pollen comes from a flower on a different plant.
Plants that have been self-pollinated and selected for type for many generations become homozygous at almost all gene loci and produce a uniform population of true breeding progeny. A cross between two different homozygous lines produces a uniform population of hybrid plants that may be heterozygous for many gene loci. A cross of two plants may each be heterozygous for many gene loci. A cross of two plants, each heterozygous at a number of gene loci, will produce a population of hybrid plants that differ genetically and will not be uniform.
There are numerous steps in the development of any novel, desirable plant germplasm. Plant breeding begins with the analysis and definition of problems and weaknesses of the current germplasm, the establishment of program goals, and the definition of specific breeding objectives. The next step is selection of germplasm that possess the traits to meet the program goals. The goal is to combine in a single variety an improved combination of desirable traits from the parental germplasm. These important traits may include higher seed yield, resistance to diseases and insects, resistance to herbicides, better stems and roots, tolerance to drought and heat, and better agronomic quality.
Choice of breeding or selection methods depends on the mode of plant reproduction, the heritability of the trait(s) being improved, and the type of cultivar used commercially (e.g., F1 hybrid cultivar, pure line cultivar, etc.). For highly heritable traits, a choice of superior individual plants evaluated at a single location will be effective, whereas for traits with low heritability, selection should be based on mean values obtained from replicated evaluations of families of related plants. Popular selection methods commonly include pedigree selection, modified pedigree selection, mass selection, and recurrent selection.
The complexity of inheritance influences choice of the breeding method. Backcross breeding is used to transfer one or a few favorable genes for a highly heritable trait into a desirable cultivar. This approach has been used extensively for breeding disease-resistant cultivars. Various recurrent selection techniques are used to improve quantitatively inherited traits controlled by numerous genes. The use of recurrent selection in self-pollinating crops depends on the ease of pollination, the frequency of successful hybrids from each pollination, and the number of hybrid offspring from each successful cross.
Each breeding program should include a periodic, objective evaluation of the efficiency of the breeding procedure. Evaluation criteria vary depending on the goal and objectives, but should include gain from selection per year based on comparisons to an appropriate standard, overall value of the advanced breeding lines, and number of successful cultivars produced per unit of input (e.g., per year, per dollar expended, etc.).
Sunflower (Helianthus annuus L.), can be bred by both self-pollination and cross-pollination techniques. The sunflower head (inflorescence) usually is composed of about 1,000 to 2,000 individual disk flowers joined to a common base (receptacle). The flowers around the circumference are ligulate ray flowers with neither stamens nor pistil. The remaining flowers are hermaphroditic and protandrous disk flowers.
Natural pollination of sunflower occurs when flowering starts with the appearance of a tube partly exerted from the sympetalous corolla. The tube is formed by the five syngenesious anthers, and pollen is released on the inner surface of the tube. The style lengthens rapidly and forces the stigma through the tube. The two lobes of the stigma open outward and are receptive to pollen but out of reach of their own pollen initially. Although this largely prevents self-pollination of individual flowers, flowers are exposed to pollen from other flowers on the same head by insects, wind and gravity.
Promising advanced breeding lines are thoroughly tested and compared to appropriate standards in environments representative of the commercial target area(s) for three or more years. The best lines are candidates for new commercial cultivars; those still deficient in a few traits may be used as parents to produce new populations for further selection.
These processes, which lead to the final step of marketing and distribution, usually take from eight to 12 years from the time the first cross is made. Therefore, development of new cultivars is a time-consuming process that requires precise forward planning, efficient use of resources, and a minimum of changes in direction.
A difficult task is the identification of individuals that are genetically superior because, for most traits, the true genotypic value is masked by other confounding plant traits or environmental factors. One method of identifying a superior plant is to observe its performance relative to other experimental plants and to a widely grown standard cultivar. If a single observation is inconclusive, replicated observations provide a better estimate of its genetic worth.
The goal of plant breeding is to develop new, unique and superior sunflower cultivars and hybrids. The breeder initially selects and crosses two or more parental lines, followed by repeated selfing and selection, producing many new genetic combinations. The breeder can theoretically generate billions of different genetic combinations via crossing, selfing and mutations. The breeder has no direct control at the cellular level. Therefore, two breeders will never develop the same line, or even very similar lines, having the same sunflower traits.
Each year, the plant breeder selects the germplasm to advance to the next generation. This germplasm is grown under unique and different geographical, climatic and soil conditions and further selections are then made, during and at the end of the growing season. The cultivars that are developed are unpredictable. This unpredictability is due to the breeder's selection, which occurs in unique environments, with no control at the DNA level (using conventional breeding procedures), and with millions of different possible genetic combinations being generated. A breeder of ordinary skill in the art cannot predict the final resulting lines he develops, except possibly in a very gross and general fashion. The same breeder cannot produce the same cultivar twice by using the exact same original parents and the same selection techniques. This unpredictability results in the expenditure of large amounts of research monies to develop superior new sunflower cultivars.
The development of new sunflower cultivars requires the development and selection of sunflower varieties, the crossing of these varieties, and selection of superior hybrid crosses. The hybrid seed is produced by manual crosses between selected male-fertile parents or by using male sterility systems. These hybrids are selected for certain single gene traits such as pod color, flower color, pubescence color, or herbicide resistance, which indicate that the seed is truly a hybrid. Additional data on parental lines, as well as the phenotype of the hybrid, influence the breeder's decision whether to continue with the specific hybrid cross.
Pedigree breeding and recurrent selection breeding methods are used to develop cultivars from breeding populations. Breeding programs combine desirable traits from two or more cultivars or various broad-based sources into breeding pools from which cultivars are developed by selfing and selection desired phenotypes. The new cultivars are evaluated to determine which have commercial potential.
Pedigree breeding is used commonly for the improvement of self-pollinating crops. Two parents which possess favorable, complementary traits are crossed to produce an F1. An F2 population is produced by selfing one or several F1s. Selection of the best individuals may begin in the F2 population; then, beginning in the F3, the best individuals in the best families are selected. Replicated testing of families can begin in the F4 generation to improve the effectiveness of selection for traits with low heritability. At an advanced stage of inbreeding (i.e., F6 and F7), the best lines or mixtures of phenotypically similar lines are tested for potential release as new cultivars.
Mass and recurrent selection can be used to improve populations of either self- or cross-pollinating crops. A genetically variable population of heterozygous individuals is either identified or created by intercrossing several different parents. The best plants are selected based on individual superiority, outstanding progeny, or excellent combining ability. The selected plants are intercrossed to produce a new population in which further cycles of selection are continued.
Backcross breeding has been used to transfer genes for a simply inherited highly heritable trait into a desirable homozygous cultivar or inbred line, which is the recurrent parent. The source of the trait to be transferred is called the donor parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent. After the initial cross, individuals possessing the phenotype of the donor parent are selected and repeatedly crossed (backcrossed) to the recurrent parent. The resulting plant is expected to have the attributes of the recurrent parent (e.g., cultivar) and the desirable trait transferred from the donor parent.
The single-seed descent procedure in the strict sense refers to planting a segregating population, harvesting a sample of one seed per plant, and using the one-seed sample to plant the next generation. When the population has been advanced from the F2 to the desired level of inbreeding, the plants from which lines are derived will each trace to different F2 individuals. The number of plants in a population declines each generation due to failure of some seeds to germinate or some plants to produce at least one seed. As a result, not all of the F2 plants originally sampled in the population will be represented by a progeny when generation advance is completed.
In a multiple-seed procedure, sunflower breeders commonly harvest seeds from each plant in a population and thresh them together to form a bulk. Part of the bulk is used to plant the next generation and part is put in reserve. The procedure has been referred to as a “modified single-seed descent.”
The multiple-seed procedure has been used to save labor at harvest. It is considerably faster to remove seeds with a machine than to remove one seed from each by hand for the single-seed procedure. The multiple-seed procedure also makes it possible to plant the same number of seeds of a population each generation of inbreeding. Enough seeds are harvested to make up for those plants that did not germinate or produce seed.
Descriptions of other breeding methods that are commonly used for different traits and crops can be found in one of several reference books (e.g., Allard, 1960; Simmonds, 1979; Sneep et al., 1979; Fehr, 1987), the contents of which are incorporated herein by this reference.
Proper testing should detect any major faults and establish the level of superiority or improvement over current cultivars. In addition to showing superior performance, there must be a demand for a new cultivar that is compatible with industry standards or which creates a new market. The introduction of a new cultivar can incur additional costs to the seed producer, the grower, processor and consumer due to special advertising and marketing, altered seed and commercial production practices, and new product utilization. The testing preceding release of a new cultivar should take into consideration research and development costs as well as technical superiority of the final cultivar. For seed-propagated cultivars, it must be feasible to produce seed easily and economically.
Once the inbred plants that give the best hybrid performance have been identified, the hybrid seed can be reproduced indefinitely as long as the homogeneity of the inbred parent is maintained. A single-cross hybrid is produced when two inbred lines are crossed to produce the F1 progeny. A double-cross hybrid is produced from four inbred lines crossed in pairs (A×B and C×D) and then the two F1 hybrids are crossed again (A×B)×(C×D). Much of the hybrid vigor exhibited by F1 hybrids is lost in the next generation (F2). Consequently, seed from hybrid varieties is not used for planting stock.
The very rapid expansion over the last decade of acreage planted with sunflower in the United States is due in part to several important developments in the field of sunflower breeding and varietal improvement. One significant development was the discovery of cytoplasmic male sterility and genes for fertility restoration, a discovery that allowed the production of hybrid sunflowers. The hybrids thus produced were introduced during the early 1970s. A description of cytoplasmic male sterility (CMS) and genetic fertility restoration in sunflowers is presented by Fick, “Breeding and Genetics,” in Sunflower Science and Technology 279-338 (J. F. Carter, ed., 1978), the contents of which are incorporated herein by this reference.
A reliable method of controlling male fertility in plants offers the opportunity for improved plant breeding. This is especially true for development of sunflower hybrids, which relies upon some sort of male sterility system. Two types of male sterility, genetic and cytoplasmic, have been found in sunflower. The use of male sterility in plant breeding has been described in U.S. Pat. No. 6,956,156, the contents of which are incorporated herein by this reference.
Sunflower, Helianthus annuus L., is an important and valuable field crop. Thus, a continuing goal of plant breeders is to develop stable, high-yielding sunflower cultivars that are agronomically sound. A current goal is to maximize the amount of grain produced on the land used and to supply food for both animals and humans. To accomplish this goal, the sunflower breeder must select and develop sunflower plants that have traits that result in superior cultivars.
Weed species have long been a problem in cultivated fields. Although once a labor intensive operation, weed control has been made easier by the availability of efficient weed killing chemical herbicides. The wide-spread use of herbicides, along with improved crop varieties and fertilizers, has made a significant contribution to the “green revolution” in agriculture. Not all herbicides are capable of selectively targeting the undesirable plants over crop plants, as well as being non-toxic to animals. Often it is necessary to settle for compounds that are simply less toxic to crop plants than to weeds. Particularly useful herbicides are those that have a broad spectrum of herbicidal activity. Unfortunately, broad spectrum herbicides typically have deleterious effect on crop plants exposed to the herbicide. As such, the development of herbicide resistant crop plants has become a major focus of agricultural research.
One particular broad spectrum herbicide that has been investigated is imidazolinone. The imidazolinone herbicides include: imazapyr, imazapic, imazethapyr, imazamox, imazamethabenz, and imazaquin. These herbicides control weeds by disrupting the activity of the enzyme acetohydroxyacid synthase (AHAS), also called acetolactate synthase (ALS). AHAS is a critical enzyme for the biosynthesis of branched chain amino acids in plants, as is described in Tan et al. (2005), Imidazolinone-tolerant crops: history, current status, and future. Pest Management Science, vol. 61: pp. 246-257, the contents of which are incorporated herein by this reference. There are several variant AHAS genes that have conferred imidazolinone tolerance and have been used to create various imidazolinone-tolerant crops, as has been described in U.S. Pat. Nos. 5,767,361, and 4,761,373, which are both incorporated herein by this reference. Mutations in the AHAS coding regions alter the enzyme structure and prevent inhibition of the enzyme by the herbicide. Tolerance to broad spectrum herbicides provides an economically viable method to control a wide range of weeds in domesticated crops.
Disease in plants is caused by biotic and abiotic causes. Biotic causes of disease include fungi, viruses, bacteria, and nematodes. Of these, fungi are the most frequent causative agent of disease in plants. The fungus causing downy mildew of cultivated sunflowers, also known as Plasmopara halstedii is a major pathogen affecting domesticated sunflower crops. The various hosts for downy mildew have been described in Leppik, E. E. (1966) Origin and specialization of Plasmopara halstedii complex on Compositae. FAO Plant Protection Bulletin 14, 72-76, and Novotel'nova, N. S. (1966) [Downy mildew of sunflower], 150 pp. Nauka, Moscow, Russia, the contents of which are incorporated herein by this reference. Downy mildew is a soil-borne pathogen, inoculating young sunflower seedlings primarily with its oospores. There is also the chance for wind-borne infection that is spread via sporangia, or can be seed-borne from infected plants, but usually only leads to limited localized infection.
The symptoms of infection with downy mildew depends on the age of the plant tissue, level of inoculum, environmental conditions (moisture and temperature) and cultivar reaction. The main symptoms are damping-off seedlings, systemic infection of stem, leaves, and flower/seed head, which is the most typical, and important, cotyledon-limited system infection, localized below-ground infection of roots and/or hypocotyls, and localized leaf infections causing angular leaf spotting. Sunflowers systemically infected with downy mildew are stunted and the leaves show characteristic green and chlorotic mottling along the leaf veins and over the lamella. When conditions are wet, a white downy growth appears on the lower leaf surface, as is further described in Zimmer, D. E. and Hoes, J. A. (1978) Diseases. In: Sunflower science and technology (Ed. by Carter, J. F.), pp. 225-262. American Society of Agronomy, Madison, USA; as well as in Sackston, W. E. (1981) Downy mildew of sunflower. In: The downy mildews (Ed. by Spencer, D. M.), pp. 545-575. Academic Press, London, UK, the contents of which are incorporated herein by this reference. The economic impact of this parasite is the significant reduction in yield of infected crops due to premature death, reduction in overall seed production and severely mildewed seedlings. Sackston described that after downy mildew first appeared in Europe in 1941, it took only 36 years for it to be rated a “major disease” in all sunflower-producing countries of Europe.
The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification.